Most scintillators known in the prior art are implemented in wide-gap insulating materials doped (“activated”) with radiation centers. A classical example of a solid-state scintillator is sodium iodide activated with thallium (NaI:Tl), introduced by Hofstadter more than 60 years ago. Because of the much longer wavelength of the scintillation associated with the activator energy levels, compared to the interband absorption threshold, the insulating scintillators are very transparent to their own luminescence. However, this advantage comes at a price in the transport of carriers to the activator site. Individual carriers have a poor mobility in insulators and transport efficiency requires that the generated electrons and holes form excitons and travel to the radiation site as neutral entities. The energy resolution even in the best modern scintillators does not compare well with that in semiconductors. One of the fundamental reasons for poor resolution is that the luminescent yield in dielectric scintillators is controlled by reactions that are nonlinear in the density of generated electron-hole pairs, such as the formation of excitons at low densities and the Auger recombination at high densities.
Such nonlinear processes do not exist in direct-gap doped semiconductors, where interaction with gamma radiation induces minority carriers while the concentration of majority carriers does not measurably change. Every reaction on the way to luminescence, including Auger recombination, is linear with respect to the concentration of minority carriers. One can therefore expect that doped semiconductor scintillators will not exhibit effects of non-proportionality and their ultimate energy resolution could be on par with that of diode detectors implemented in the same material.
Typically, scintillators are not made of semiconductor materials. The key issue in implementing a semiconductor scintillator is how to make the material transmit its own infrared luminescence, so that photons generated deep inside the semiconductor slab could reach its surface without tangible attenuation. However, semiconductors are usually opaque at wavelengths corresponding to their radiative emission spectrum. The inventors have been working on the implementation of radiation detectors based on direct-gap semiconductor scintillator wafers, like InP or GaAs. For the exemplary case of InP the scintillation spectrum is a band of wavelengths near 920 nm. The initial approach was to make InP relatively transparent to this radiation by doping it heavily with donor impurities, so as to introduce the Burstein shift between the emission and the absorption spectra. Because of the heavy doping, the edge of absorption is blue-shifted relative to the emission edge by the carrier Fermi energy. However, Burstein's shift by itself does not provide adequate transparency at room temperature. The problem is that attenuation of the signal depends on depth of the interaction site into the semiconductor (see Serge Luryi and Arsen Subashiev, “Semiconductor Scintillator for 3-Dimensional Array of Radiation Detectors” in Future Trends in Microelectronics: From Nanophotonics to Sensors to Energy, ed. by S. Luryi, J. M. Xu, and A. Zaslaysky, Wiley Interscience, Hoboken, N.J. (2010) pp. 331-346.)
The transparency issue is of critical importance and one is concerned with new ways to enhance the photon delivery to the semiconductor surface.
One possibility is to implement a semiconductor version of activated scintillator, similar in principle to NaI:Tl, by doping the semiconductor with high efficiency radiative centers that emit below-bandgap light. It is important that the excited electron-hole pairs be efficiently transferred to the radiative center. In the case of InP, this energy transfer probability was shown to be high for certain trivalent luminescent ions incorporated in the host lattice. The system InP:Yb3+ seems to work at cryogenic temperatures, producing emission near 1 μm—well below the bandgap of InP. However, at room temperature, its performance is degraded by fast non-radiative de-excitation of Yb ions.
Other ideas for implementing transparent semiconductor scintillators include replacing luminescent ions by semiconductor wells or “impregnations” of lower bandgap. This idea was proposed in Kastalsky, Luryi, et al. publication, (see “Semiconductor high-energy radiation scintillation detector,” Nucl. Instr. and Meth. in Phys. Research A 565, pp. 650-656 (2006) and in U.S. Pat. No. 7,265,354 to Kastalsky et al. and further discussed by Luryi (see “Impregnated Semiconductor Scintillator,” International Journal of High Speed Electronics and Systems, vol. 18, No 4 pp. 973-982 (2008)).
The epitaxially grown structure comprises two alternating materials that are lattice-matched to each other. The materials are assumed to have different energy gaps, with the second material having the lower bandgap, EG1>EG2. The essential idea is that the total volume occupied by the second material is small compared to that occupied by the first material. The ratio of these volumes defines a “duty cycle” factor S and the absorption coefficient of the composite structure is reduced by this factor. For example, if a 2 μm-thick InP layers are alternated by a 20 nm-thick layers of InGaAsP, the volume ratio is 100 (δ=0.01).
We are referring now to FIG. 1 which is a schematic band diagram of the prior-art detector as disclosed by Kastalsky et al. The layered detector comprises a sequence of alternating barrier layers 11 and well layers 12 where the thickness of barrier layers is limited to about 2 μm by the requirement that it must be much smaller than the diffusion length of minority carriers. The band diagram of FIG. 1 shows the conduction band edges ECi and the valence band edges EVi of the constituent materials, where i=1 refers to the barrier material and i=2 refers to the well material. Also shown in FIG. 1 is the Fermi level EF position of which in the band diagram corresponds to n-type doped semiconductor.
The crucial requirement for the structure disclosed by Kastalsky et al. publication and patent and illustrated in FIG. 1 is that the distance between the narrow-gap wells must be shorter than the diffusion length LD of carriers in the wide-gap material, which guarantees that most of the light emission occurs in the wells. This requirement limits the separation between wells to a few microns and is very hard to implement in practice, where one is interested in thick scintillator structures, exemplarily one millimeter thick. There are crystal growth techniques, like Vapor Phase Epitaxy, that offer fast growth of semiconductor layers, but these techniques are largely limited to growth of homogeneous layers like GaAs or InP. Rapid growth of short-period (several microns) superlattices required by Kastalsky et al. publication and patent is very difficult.
The short-period requirement in prior-art layered semiconductor scintillators results from the need to capture into the lower-gap wells most of the minority carriers generated in the wide-gap material. The present invention circumvents this requirement. As will be fully explained below, no travel of minority carriers is contemplated in the inventive structure. This is a radical departure from all prior art of scintillators endowed with special radiation sites that emit light at subband wavelengths. In all prior art scintillators, charge carriers were supposed to travel to these radiation sites and the distance to travel had to be minimized by increasing the concentration of radiation sites. Nevertheless, the finite travel distance leads to the above-mentioned non-proportionality of activated dielectric scintillators.
In the inventive structure, the minority carriers generated in the wide-gap material recombine there radiatively and the short-wavelength light thus generated is captured by the narrow-gap wells generating new minority carriers therein. Recombination of these new minority carriers in the narrow-gap wells generates longer wavelength scintillation, to which the entire layered structure is largely transparent. It is important that the separation between narrow-gap wells is no longer limited by the minority-carrier diffusion length and can be as large as hundreds of microns. The actual limitation on the well separation, according to the present invention, results from the need to capture most of the short-wavelength light by the narrow-gap wells. That in turn leads to an optimization strategy for the choice of the wide-gap host material and its doping. The primary requirement is high radiative efficiency of the host material. As will be fully explained below, the loss of short-wavelength light on the way to the narrow-gap wells is associated only with free-carrier absorption. The interband absorption of a photon merely generates a new minority carrier, which again recombines radiatively to produce another photon. The high radiative efficiency requirement ensures that non-radiative channels of recombination of minority carriers are minimized.